Standard STP convergence time?

Discussion in 'Cisco' started by aaabbb16@hotmail.com, Jun 26, 2008.

  1. Guest

    stp convergence time =50s or 30-50s? why?

    TIA,
    st
     
    , Jun 26, 2008
    #1
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  2. wrote:
    > stp convergence time =50s or 30-50s? why?


    For regular STP with default timers on Cisco equipment and properly
    configured ports - it is 30 seconds.

    Regards,
    Andrey.
     
    Andrey Tarasov, Jun 26, 2008
    #2
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  3. Guest

    On 26 Jun, 05:26, Andrey Tarasov <> wrote:
    > wrote:
    > > stp convergence time =50s or 30-50s? why?

    >
    > For regular STP with default timers on Cisco equipment and properly
    > configured ports - it is 30 seconds.
    >
    > Regards,
    > Andrey.


    Hmmm. I would disagree with that.

    I too was somewhat confused by observing that re-convergence
    can take either about 30 seconds or about 45 seconds (maybe 35 and
    50?).

    I believe that it is like this.

    For a port that is not connected that gets connected STP has to go
    through
    all of the states

    Blocking
    Listening
    Learning
    Forwarding
    45-50 seconds - If I recall correctly

    For a port that is already in the network and is Blocking
    The transition to Forwarding is about 30-35 seconds

    The default timers will be easy to find on the web.

    Note that the times do not really range over the interval indicated
    but I have specified a range since I cannot remember the correct
    times and BPDUs are sent only every two seconds so there will
    be a range of times introduced by the port state change possibly
    occurring just before or just after a BPDU tx or rx event.

    Of course there are other considerations.

    For a newly connected port then there may be
    physical speed/duplex negotiation
    etherchannel negotiation
    trunking negotiation

    There are also various STP optimisations
    portfast
    uplink fast
    backbone fast

    Lastly of course there is Rapid STP which is substantially changed
    from
    traditional STP and has much lower convergence times. I have not
    worked
    with it but have the idea that convergence times of one to a few
    seconds are
    reasonable.
     
    , Jun 26, 2008
    #3
  4. Guest

    On 6ÔÂ26ÈÕ, ÉÏÎç6ʱ24·Ö, wrote:
    > On 26 Jun, 05:26, Andrey Tarasov <> wrote:
    >
    > > wrote:
    > > > stp convergence time =50s or 30-50s? why?

    >
    > > For regular STP with default timers on Cisco equipment and properly
    > > configured ports - it is 30 seconds.

    >
    > > Regards,
    > > Andrey.

    >
    > Hmmm. I would disagree with that.
    >
    > I too was somewhat confused by observing that re-convergence
    > can take either about 30 seconds or about 45 seconds (maybe 35 and
    > 50?).
    >
    > I believe that it is like this.
    >
    > For a port that is not connected that gets connected STP has to go
    > through
    > all of the states
    >
    > Blocking
    > Listening
    > Learning
    > Forwarding
    > 45-50 seconds - If I recall correctly
    >
    > For a port that is already in the network and is Blocking
    > The transition to Forwarding is about 30-35 seconds
    >
    > The default timers will be easy to find on the web.
    >
    > Note that the times do not really range over the interval indicated
    > but I have specified a range since I cannot remember the correct
    > times and BPDUs are sent only every two seconds so there will
    > be a range of times introduced by the port state change possibly
    > occurring just before or just after a BPDU tx or rx event.
    >
    > Of course there are other considerations.
    >
    > For a newly connected port then there may be
    > physical speed/duplex negotiation
    > etherchannel negotiation
    > trunking negotiation
    >
    > There are also various STP optimisations
    > portfast
    > uplink fast
    > backbone fast
    >
    > Lastly of course there is Rapid STP which is substantially changed
    > from
    > traditional STP and has much lower convergence times. I have not
    > worked
    > with it but have the idea that convergence times of one to a few
    > seconds are
    > reasonable.


    Thanks,
    I think 50s=2x15s(forwarding delay)+20s(max age time)
    so the max. convergence time is 50s and min. is 30s. right?
    why does max age time is from 0-20s?

    st
     
    , Jun 26, 2008
    #4
  5. wrote:
    > On 26 Jun, 05:26, Andrey Tarasov <> wrote:
    >> wrote:
    >>> stp convergence time =50s or 30-50s? why?

    >> For regular STP with default timers on Cisco equipment and properly
    >> configured ports - it is 30 seconds.
    >>
    >> Regards,
    >> Andrey.

    >
    > Hmmm. I would disagree with that.


    That's fine. How about some facts?

    > I too was somewhat confused by observing that re-convergence
    > can take either about 30 seconds or about 45 seconds (maybe 35 and
    > 50?).
    >
    > I believe that it is like this.
    >
    > For a port that is not connected that gets connected STP has to go
    > through
    > all of the states
    >
    > Blocking
    > Listening
    > Learning
    > Forwarding
    > 45-50 seconds - If I recall correctly


    Why? 2 x forwarding delay is 28.5 sec. Where are additional 15-20 sec
    coming from?

    >
    > For a port that is already in the network and is Blocking
    > The transition to Forwarding is about 30-35 seconds


    So what's the difference between newly connected vs already connected?

    > The default timers will be easy to find on the web.
    >
    > Note that the times do not really range over the interval indicated
    > but I have specified a range since I cannot remember the correct
    > times and BPDUs are sent only every two seconds so there will
    > be a range of times introduced by the port state change possibly
    > occurring just before or just after a BPDU tx or rx event.
    >
    > Of course there are other considerations.
    >
    > For a newly connected port then there may be
    > physical speed/duplex negotiation
    > etherchannel negotiation
    > trunking negotiation


    Not relevant to original question. While etherchannel/trunking
    negotiation indeed adds time, it's not part of STP convergence.

    >
    > There are also various STP optimisations
    > portfast


    That's enabled on edge ports only and doesn't affect convergence time at
    all.

    > uplink fast
    > backbone fast


    Cisco extensions and not part of original 802.1D spec.

    > Lastly of course there is Rapid STP which is substantially changed
    > from
    > traditional STP and has much lower convergence times. I have not
    > worked
    > with it but have the idea that convergence times of one to a few
    > seconds are
    > reasonable.


    Again, Rapid STP is completely different beast.

    Regards,
    Andrey.
     
    Andrey Tarasov, Jun 26, 2008
    #5
  6. Andrey Tarasov wrote:
    > wrote:
    >> On 26 Jun, 05:26, Andrey Tarasov <> wrote:
    >>> wrote:
    >>>> stp convergence time =50s or 30-50s? why?
    >>> For regular STP with default timers on Cisco equipment and properly
    >>> configured ports - it is 30 seconds.
    >>>
    >>> Regards,
    >>> Andrey.

    >>
    >> Hmmm. I would disagree with that.

    >
    > That's fine. How about some facts?
    >
    >> I too was somewhat confused by observing that re-convergence
    >> can take either about 30 seconds or about 45 seconds (maybe 35 and
    >> 50?).
    >>
    >> I believe that it is like this.
    >>
    >> For a port that is not connected that gets connected STP has to go
    >> through
    >> all of the states
    >>
    >> Blocking
    >> Listening
    >> Learning
    >> Forwarding
    >> 45-50 seconds - If I recall correctly

    >
    > Why? 2 x forwarding delay is 28.5 sec. Where are additional 15-20 sec
    > coming from?
    >
    >>
    >> For a port that is already in the network and is Blocking
    >> The transition to Forwarding is about 30-35 seconds

    >
    > So what's the difference between newly connected vs already connected?
    >
    >> The default timers will be easy to find on the web.
    >>
    >> Note that the times do not really range over the interval indicated
    >> but I have specified a range since I cannot remember the correct
    >> times and BPDUs are sent only every two seconds so there will
    >> be a range of times introduced by the port state change possibly
    >> occurring just before or just after a BPDU tx or rx event.
    >>
    >> Of course there are other considerations.
    >>
    >> For a newly connected port then there may be
    >> physical speed/duplex negotiation
    >> etherchannel negotiation
    >> trunking negotiation

    >
    > Not relevant to original question. While etherchannel/trunking
    > negotiation indeed adds time, it's not part of STP convergence.
    >
    >>
    >> There are also various STP optimisations
    >> portfast

    >
    > That's enabled on edge ports only and doesn't affect convergence time at
    > all.
    >
    >> uplink fast
    >> backbone fast

    >
    > Cisco extensions and not part of original 802.1D spec.
    >
    >> Lastly of course there is Rapid STP which is substantially changed
    >> from
    >> traditional STP and has much lower convergence times. I have not
    >> worked
    >> with it but have the idea that convergence times of one to a few
    >> seconds are
    >> reasonable.

    >
    > Again, Rapid STP is completely different beast.
    >
    > Regards,
    > Andrey.


    Andrey,

    to sum it up. Going through all stp states takes 30 seconds! Neither less nor
    more :)

    Andre
     
    Andre Wisniewski, Jun 28, 2008
    #6
  7. John Agosta Guest

    <> wrote in message
    news:...
    > stp convergence time =50s or 30-50s? why?
    >
    > TIA,
    > st



    Let's say the lowest cost root path includes 3 switches between you and the
    root,
    and there is also an inferior path to the root with higher cost.

    If something should affect the continuity between , say, the root and the
    first switch closest to the
    root on your "preferred" path, your switch will not detect the failure until
    20 seconds pass
    without seeing any of the 'preferred' bpdus. After realizing there is a
    failure (20 seconds),
    your switch will then go through the normal 15 listening/15 learning second
    convergence.
    But that doesn't happen until your switch realizes there is a problem, which
    too 20 seconds
    of no-show bpdus up front. Total time - 50 seconds.
     
    John Agosta, Jun 28, 2008
    #7
  8. donjohnston

    Joined:
    Jun 28, 2008
    Messages:
    38
    But don't forget that with Cisco, if the root port interface goes down, the switch doesn't wait for the Max Age timer to expire. The blocked port will immediately transition to "Listening" and then "Learning" which results in a 30 second convergence.

    If the interface does not go down and BPDU's are not received on the port, then you have to wait for the Max Age timer to expire first which results in a 50 second convergence.
     
    donjohnston, Jun 28, 2008
    #8
  9. John Agosta wrote:
    > <> wrote in message
    > news:...
    >> stp convergence time =50s or 30-50s? why?
    >>
    >> TIA,
    >> st

    >
    >
    > Let's say the lowest cost root path includes 3 switches between you and the
    > root,
    > and there is also an inferior path to the root with higher cost.
    >
    > If something should affect the continuity between , say, the root and the
    > first switch closest to the
    > root on your "preferred" path, your switch will not detect the failure until
    > 20 seconds pass
    > without seeing any of the 'preferred' bpdus. After realizing there is a
    > failure (20 seconds),
    > your switch will then go through the normal 15 listening/15 learning second
    > convergence.
    > But that doesn't happen until your switch realizes there is a problem, which
    > too 20 seconds
    > of no-show bpdus up front. Total time - 50 seconds.


    My bad. Indirect failure will indeed produce 50 seconds convergence.
    Convergence for direct failure is 30 sec though.

    Regards,
    Andrey.
     
    Andrey Tarasov, Jun 28, 2008
    #9
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